1. Field of the Invention
The present invention relates to a method of forming a uniform, stable, low strain native oxide layer on a compound semiconductor material. More specifically, the present invention forms the native oxide layer by a method of pulsed anodic oxidation at room temperature. The oxide layer forms much faster than the oxide layer formed from the prior art continuous oxidization, and has the feature of being able to travel into the compound semiconductor material. High quality oxides can be formed on compound semiconductor materials, and in particular, on gallium, aluminum, and indium containing compound semiconductor materials.
The present invention is also directed to devices using the native oxide layer, including electrical and optoelectronic devices such as transistors, capacitors, waveguides, and especially lasers.
The present invention relates to the masking and passivation of compound semiconductors using the native oxide that forms from the practice of present invention.
2. Description of the Prior Art
Silicon technology has far outstripped other semiconductor technologies because the silicon oxide can be produced cheaply and easily, and exhibits outstanding surface state and electrical properties. Group III-V semiconductors have been limited to uses where their high speed and ability to produce light are absolutely necessary. Group II-VI semiconductors have recently assumed importance because new and short wavelength lasers have been produced using ZnSe-based materials. An inexpensive and simple method of producing a high quality native oxide on such semiconductors would allow inexpensive dielectric insulation and isolation of devices from one another, and surface passivation, which would broaden the uses of such material.
A schematic drawing of the prior art fabrication procedure for a monolithic array of semiconductor laser diodes is shown in FIG. 1. A substrate 2 of n-type GaAs material has a number of layers grown epitaxially. The important cladding layers 6 of n-type AlGaAs and 7 of p-type AlGaAs surround an active layer 4 which normally has a lower bandgap than the cladding layers and may be composed of many layers of AlGaAs, GaAs, InGaAs, or other III-V semiconductor compounds. A layer of exposed and developed photoresist 8 is shown with openings 9 etched in the photoresist. Step b of the prior art procedure uses an etching step to etch vee grooves in the cladding layer 7, generally through the active layer 4 and into the other cladding layer 6. A blanket layer of insulating material 12 such as SiO2 is then deposited on the substrate. This step generally requires heating the wafer which can increase the defects generated by heating and cooling the many layers of different material. Another layer of photoresist is then deposited on the wafer, and is exposed and developed in an expensive alignment procedure to give the photoresist portions 14 covering the vee grooves 10. The SiO2 is then etched away from the areas between the vee grooves 10, the photoresist is stripped, and blanket metalization layers 18 and 20 are deposited over the front side and the back side of the wafer. The remaining oxide isolates each laser diode from its neighbor. In contrast, the fabrication of a monolithic array of semiconductor laser diodes using the method of the present invention is shown in FIG. 2, where pulsed anodic oxidization is used to produce trenches 22 covered with native oxide 24, saving the expensive lithography step and reducing the defect causing temperature excursions of the prior art.
Wet, high temperature methods of forming native oxides are retorted to in U.S. Pat. No. 5,262,360, hereby incorporated by reference, which patent teaches a method of wet chemical oxidation of aluminum containing III-V semiconductors at temperatures of greater than 375C. The thickness of the native oxide is substantially the same as, or less than, the thickness of that portion of the material converted to oxide. The index of refraction of the oxide material is about 1.57.
C. W. Fischer and S. W. Teare, in J. Appl. Phys. 67, 2608, (1990) have reported on the physical properties and growth mechanisms of anodic oxides on GaAs, and shown the advantages of using a glycol:water:acid (GWA) solution. Prior art publications suggest that native oxides produced by anodic oxidation on III-V semiconductors are unstable in the various washing, solvent materials, photoresist strippers, etc needed for modem device processing. R. S. Burton et. al., Appl. Phys. Lett. 60, 1776, (1990) is an example of a report that anodic oxides are apparently unstable with respect to subsequent processing. A. Sher el. al., in J. Electronic Materials 23, 653, (1992) have reported on fabrication of native oxide on ZnTe by anodic oxidation in water and methanolic solutions. T. Suda et. al. Jap. J. Appl. Phys. 25, L162, (1986) have reported on fabrication of native oxide on Zn.sub.3 P.sub.2 by anodic oxidation in a 3% tartaric acid/propylene/glycol electrolyte. K. Yokoyama et. al. in Plating and Surface finishing page 62, July, (1982) have reported producing porous aluminum oxide material on aluminum alloys by changing the anodizing voltage by a maximum of 30% for times ranging from 10 seconds to 60 seconds. C. Tu and L. Y. Huang, Trans. IMF 65, 60 (1987) have reported cracked and rough surfaces using 60 Hz AC and DC pulses when anodizing 2024 Aluminum alloy
The semiconductor art has produced a number of methods to form oxides of compound semiconductors, and recognizes a continuing need for a method of growing an improved, high quality native oxide compound semiconductors. The method should be simple, cost effective, low temperature, and produce the native oxide in a consistent and repeatable manner.